1. Field of the Invention
This invention relates to electrical devices which comprise a conductive polymer composition.
2. Introduction to the Invention
Conductive polymer compositions and devices comprising them are known. They comprise a particulate conductive filler which is dispersed in, or otherwise held together by, an organic polymer. They can be used in circuits in which current passes through them, e.g. in heaters and circuit protection devices, and in such use they may exhibit what is known as PTC (positive temperature coefficient) or ZTC (zero temperature coefficient) behavior. The term "PTC behavior" is usually used in the art, and is so used in this specification, to denote a composition which, in the operating temperature range, has an R.sub.14 value of at least 2.5 or an R.sub.100 value of at least 10, preferably both, and which preferably has an R.sub.30 value of at least 6, where R.sub.14 is the ratio of the resistivities at the end and the beginning of the 14.degree. C. temperature range showing the greatest increase to resistivity, R.sub.100 is the ratio of the resistivities at the end and the beginning of the 100.degree. C. temperature range showing the greatest increase in resistivity, and R.sub.30 is the ratio of the resistivities at the end and the beginning of the 30.degree. C. temperature range showing the greatest increase in resistivity. The term "ZTC behavior" is usually used in the art, and is so used in this specification, to denote a composition which does not show PTC behavior in the operating temperature range; thus the term is used to include (a) compositions which show no substantial change in resistivity over the operating temperature range (e.g. from room temperature to 100.degree. C.), (b) compositions which show substantial increases in resistivity over the operating temperature range but still do not have R.sub.14, R.sub.30 or R.sub.100 values as specified above, (c) compositions which show substantial decreases in resistivity over the operating temperature range [often denoted NTC (negative temperature coefficient) compositions], and (d) compositions as defined in (a), (b) and (c) which exhibit PTC behavior at temperatures above the operating temperature range.
Typical electrical devices comprising conductive polymer compositions comprise two electrodes, between which the conductive composition is positioned. In some known devices an intermediate member is positioned between one, or both, of the electrodes and the conductive polymer composition. Particularly relevant in this regard are U.S. Pat. No. 2,978,665 (Vernet), U.S. Pat. No. 4,177,376 (Horsma et al), U.S. Pat. No. 4,330,703 (Horsma et al) and U.S. Pat. No. 4,543,474 (Horsma et al), U.S. Pat No. 4,177,446 (Diaz et al) and U.S. Pat. No. 4,421,582 (Horsma et al), the disclosure of which are incorporated herein by reference.
The electrodes which have been used to make physical and electrical contact with conductive polymers include solid and stranded wires, metal foils, and expanded and perforated metal sheets.
A particularly useful group of devices which comprise a conductive polymer composition are those devices which exhibit so-called "PTC (positive temperature coefficient) behavior". Such devices increase in resistance as their temperature rises. When an electrical current is passed through such devices heat is generated, and the resistance of the device increases until it effectively "shuts-off" the applied current. Such devices are referred to as "self-regulating" or "self-limiting" because they automatically shut themselves off if they exceed a certain temperature. The devices find application inter alia as heater, and as circuit protection devices.
One problem associated with heaters that exhibit PTC behavior is current inrush (i.e. a much higher current passes through the device when the device is switched on than at later stages in the transition to the higher resistivity state). The high inrush current may damage the device itself and also any other components connected in the same electrical circuit as the device. Above mentioned U.S. Pat. Nos. 4,177,367, 4,330,703 and 4,543,474 inter alia alleviate the problem of high inrush currents by providing a layer of a ZTC (zero temperature coefficient) conductive polymer composition between the electrode and the conductive polymer composition.